⽤掺硼⾦刚⽯（BDD）电极的电化学氧化协同作⽤和臭氧（O3）的⼯业废⽔处理Synergy of electrochemical oxidation using boron-doped diamond (BDD)electrodes and ozone (O 3)in industrial wastewater treatmentM.A.García-Morales a ,G.Roa-Morales a ,?,Carlos Barrera-Díaz a ,Bryan Bilyeu b ,M.A.Rodrigo ca Centro Conjunto de Investigación en Química Sustentable,UAEM-UNAM,Carretera Toluca-Atlacomulco,Km 14.5,Campus San Cayetano,C.P.50200,Toluca Estado de México,Mexicob Department of Chemistry,Xavier University of Louisiana,New Orleans 70125,LA,USAcDepartment of Chemical Engineering,Facultad de Ciencias Químicas,Universidad de Castilla-La Mancha,Campus Universitario s/n 13071Ciudad Real,Spaina b s t r a c ta r t i c l e i n f o Article history:Received 9October 2012Received in revised form 22October 2012Accepted 23October 2012Available online 27October 2012Keywords:Electrooxidation Ozone BDDWastewater CODO 3-BDD coupled processThis work evaluates the coupling of electrochemical oxidation and ozonation to reduce the high organic load of industrial wastewater quickly and effectively.Ozonation alone is shown to only reduce the COD of waste-water by about45%.Electrochemical oxidation using boron-doped diamond electrodes reduces the COD by 99.9%,but requires over 2h per 0.7L batch.However,when the two processes are coupled,the COD is re-duced by 99.9%along with most color and turbidity in about an hour.The coupled process practically elimi-nates the COD,color,and turbidity without the addition of chemical reagents or changing the pH and doesn't generate any sludge,so it is both effective and environmentally friendly.2012Elsevier B.V.All rights reserved.1.IntroductionIndustrial ef ?uents are dif ?cult to treat using traditional biological systems due to the high variations in their compositions.Unlike munic-ipal wastewater,industrial sources have higher organic load,color,and pH which ?uctuate[1,2].While traditional biological reactors are very effective in digesting the organic matter in municipal wastewater into carbon dioxide and water,the effectiveness drops considerably when treating industrial wastewater.Biological reactors typically only reduce 50%of the biochemical oxygen demand (BOD 5)and 35%of the chemical oxygen demand (COD)[3,4]. Due to the limitations of biological reactors,industrial wastewater is typically pretreated using physical –chemical processes such as co-agulation –?occulation.However,these processes generate large quantities of sludge and usually require pH adjustments and chemical reagents,all of which create their own environmental issues [5,6].Co-agulation –?occulation is not ef ?cient in the removal of dissolved (persistent)chemical pollutants.In recent works we have shown that combining electrocoagulation and ozone produces synergistic effects in wastewater treatment [7,8].However,the use of electrooxidation with boron-doped diamond (BDD)electrodes in conjunction with ozone for treating industrial ef ?u-ents has not yet been reported.Both electrooxidation and ozonation are advanced oxidative pro-cesses based on the generation of hydroxyl radicals (OH ),which have high oxidation potential and degrade of a wide range of contam-inants.In particular,BDD electrodes have high anodic stability,a wide working potential window,and low stable voltammetric background current in aqueous media[9,10].Therefore,the electrochemical be-havior of BDD electrodes have been investigated with the goal of de-veloping applications for wastewater treatment [11,12].On the other hand,ozonation is an ef ?cient and powerful oxidizing process wellknown for its degradation of organic compounds.The limitations to these processes are the time required for electrooxidation and the ef-fectiveness of ozonation,so neither alone is truly industrially practical.Thus,this study evaluates the synergy of the two processes com-pared to the ef ?ciency and effectiveness of the individual ones.The effectiveness is evaluated in terms of color,turbidity and chemical ox-ygen demand (COD)reduction.The in ?uence of operating parame-ters such as time of treatment,current density,and initial pH is also evaluated.2.Materials and methods 2.1.Wastewater samplesWastewater samples were collected from the treatment plant of an industrial park,which receives the discharge of144different facil-ities.Therefore,the chemical composition of this ef ?uent is ratherElectrochemistry Communications 27(2013)34–37Corresponding author.Tel.:+527222173890;fax:+527222175109.E-mail address:groam@uaemex.mx (G.Roa-Morales).1388-2481/$–see front matter ?2012Elsevier B.V.All rights reserved./10.1016/j.elecom.2012.10.028Contents lists available at SciVerse ScienceDirectElectrochemistry Communicationsj o ur n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /e l e c o mcomplex.Samples were collected in plastic containers and cooled down to4°C,then transported to the laboratory for analysis and treatment.The pH of the raw wastewater is8.24and all treatment and testing were done at this value.2.2.Electrooxidation reactorA batch cylindrical electrochemical reactor was set up for the elec-trochemical process.The reactor cell contains a pair of BDD electrodes (BDD?lm supported on a niobium substrate),each electrode was 20.0cm by2.5cm with a surface areaof50cm2.Batch volumes of 0.70L were treated in the1.00L reactor.A direct-current power source supplied the systemwith0.5,1.0,and1.5A,corresponding to current densities of10,20,and30mA/cm2.2.3.Ozonation reactorThe ozone experiments were conducted in a1.5L glass reactor at18°C.Ozone was supplied by a Paci?c Ozone Technology generator. The gas was fed into the reactor through a porous plate situated at the reactor bottom.The ozone concentration at the gas inlet and out-let of the reactor was measured by redirecting the?ow to a series of ?asks containing0.1M potassium iodide.The mean concentration of ozone in the gas phase was5±0.5mg/L and was measured imme-diately before each run.Ozonation experiments were carried out at the pH of raw wastewater and samples were taken at regular inter-vals to determine COD.2.4.Synergy of electrooxidation/O3processFor the combined system the pair of BDD electrodes from the electrooxidation reactor was installed in the ozonereactor.Ozone was introduced at the same rate and the BDD electrodes were given the same current densities as in the individual reactors.Treated sam-ples were taken at the same intervals and were analyzed in the same way.2.5.Methods of analysisThe initial evaluations of the electrochemical,ozonation,and inte-grated treatments were determined by analysis of theCOD(mg/L), color(Pt–Co scale),and turbidity(NTU scale).COD was determined by the open re?ux method according to the American Public Health Association(APHA).Following this method,samples are re?uxed with potassium dichromate and sulfuric acid for2h.Once the opti-mal conditions were found the raw and treated wastewater samples were analyzed using the standard methods for the examination of water and wastewater procedures.[13].3.Results and discussion3.1.Electrooxidation treatmentThe COD reduction(%)as function of electrooxidation treatment time on the raw wastewater is shown in Fig.1.The maximum COD re-duction of99.9%was observed at140min of treatment.3.2.Ozonation treatmentOzone was introduced into the sample at a concentration of5±0.5mg/L and the COD was measured as a function of time.As shown in Fig.2,the maximum COD reduction was45%at120min of treatment time.3.3.Synergy of electrooxidation/O3processThe effect of coupling electrooxidation and ozonation processes was studied through a series of experiments using the COD reduction as a function of treatment time for the raw wastewater.In Fig.3the effect of variation on the current density values is also described. The maximum COD reduction of99.9%occurs at60min.A comparative graph of the COD reduction as a function of treat-ment time among the three treatments indicates that ozone is not as effective and electrooxidation takes longer than the coupled pro-cess(Fig.4).The UV–vis spectra of the raw and treated wastewater are shown in Fig.5.The raw wastewater shows considerable absorbance in the visible range of300to630nm which con?rms that it is highly col-ored.However,this color is effectively removed by the coupled treatment.The reduction in the values of some physicochemical parameters of the raw and treated wastewater is shown in Table1.As shown in Table1,the coupled process reduces and practically eliminates the organic pollutants in the wastewater.The high levels of COD,color,and turbidity are effectively reduced without any addi-tion of chemical reagents and without adjusting the pH.The coupled process also increases the ef?ciency of the organic removal by reduc-ing the treatment time.Thus the two processes act synergistically in the coupled process.Previous research[14]indicates that the oxidation of organics with concomitant oxygen evolution assumes that both organicoxidation Fig.1.COD removal as a function of electrooxidation treatment time at10mA/cm2.Fig.2.COD removal as a function of ozonation(5±0.5mg/L)treatment time.35M.A.García-Morales et al./Electrochemistry Communications27(2013)34–37and oxygen evolution take place on a BDD anode surface via intermedi-ation of hydroxyl radicals,generated from the reaction with water shown in Eqs.(1)and (2):BDD tH 2O →BDD eOH ?TtH ttee1TBDD eOH ?TtR →BDD tmCO 2tnH 2O :e2TReaction (2)is in competition with the side reaction of hydroxyl radical conversion to O 2without any participation of the anode sur-face as indicated in Eq.(3)BDD eOH ?T→BDD t1=2O 2tH tte ?:e3TThe ozone contribution can be attributed to the electrophilic na-ture of the direct attack by O 3molecules (Eq.(5))and the indirect at-tack via OH ?radicals in the ozonation process (Eq.(6)).According to Tomiyasu et al.[15]the ozonation effect may be ini-tiated by the following reactions:O 3tH 2O →2HO ?tO 2e4TO 3tOH →O ?2?tHO ?2e5TO 3tOH ?→HO ?2tO 2:e6TAccording to the literature,the pH value of the solution signi ?-cantly in ?uences ozone decomposition in water since basic pH causes an increase of ozone decomposition.At pH b 3hydroxyl radicals do not in ?uence the decomposition of ozone.For 7b pH b 10,the typical half-life of ozone is 15to 25min.[16].4.ConclusionsThe combination of electrooxidation and ozonation processes re-sults in a synergy that greatly enhances the rate and extent of remov-al of COD,color,and turbidity from a chemically complex industrial ef ?uent.Electroxodiation alone reduces the COD to less than 1%of the initial,but requires a relatively long time of 140min.On the other hand,ozonation alone only reduces it to 45%.When the coupled electrooxidation –ozonation process is used a maximum 99.9%of COD is removed in only 60min under the optimal conditions:pH 8.24,with 5±0.5mg/L of ozone concentration,and 30mA/cm 2of current density.While electrooxidation ef ?ciency usually increases with in-creasing current density,the coupled process is more ef ?cient at a rel-atively low (mA/cm 2)current density.AcknowledgmentsThe authors wish to acknowledge the support given by the Centro Conjunto de Investigación en QuímicaSustentable,UAEM-UNAM,and ?nancial support from the CONACYT through the projects 168305and 153828is greatlyappreciated.Fig.3.COD removal when coupling electrooxidation and ozonation processes at three different current densities (▲)30mA/cm 2(○)20mA/cm 2(?)10mA/cm 2.Fig.4.COD removal as a function of treatment time of (?)coupled,(Δ)electrooxidation and (■)ozonetreatment.Fig.5.UV –vis spectra of the (——)raw and (----)treated industrial wastewater.The parameters of the coupled treatment were 30mA/cm 2and 5±0.5mg/L of ozone.Table 1Physicochemical parameters of the raw and treated industrial wastewater.Parameter Raw wastewater Treated wastewater COD/mg L ?1534b 1Color/Pt –Co units 880b 50Turbidity/NTU52b 536M.A.García-Morales et al./Electrochemistry Communications 27(2013)34–37References[1] C.A.Martinez,E.Brillas,Applied Catalysis B:Environmental87(2009)105.[2] C.Barrera-Díaz,F.Ure?a-Nu?ez,E.Campos,M.Palomar-Pardavé,M.Romero-Romo,Radiation Physics and Chemistry67(2003)657.[3]V.Agridiotis,C.Forster,C.Balaboine,C.Wolter,C.Carliell-Marquet,Water Envi-ronment Journal20(2006)141.[4] C.J.Van der Gast,B.Jefferson,E.Reid,T.Robinson,M.J.Bailey,S.J.Judd,I.P.Thompson,Environmental Microbiology8(6)(2006)1048.[5] F.Hana?,O.Assobhei,M.Mountadar,Journal of Hazardous Materials174(2010)807.[6] C.Barrera-Díaz,I.Linares-Hernández,G.Roa-Morales,B.Bilyeu,P.Balderas-Hernández,Industrial and Engineering Chemistry Research48(2009)1253. 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